1. Field of the Invention
The present invention relates to straddle-type vehicles, and in particular, to a straddle-type vehicle equipped with an electronically controlled belt-type continuously variable transmission.
2. Description of Related Art
Straddle-type vehicles such as motor scooters widely employ V-belt type continuously variable transmissions comprising a pair of primary and secondary sheaves with a variable groove width. The primary sheave is disposed on a primary shaft to which the output of power source such as an engine is input. The secondary sheave is disposed on a secondary shaft that extracts the output to a driving wheel. A V-belt is wound around the sheaves. The winding diameters of the V-belt around the sheaves are controlled by changing the groove widths by a groove-width adjusting mechanism so that the speed change ratio of the sheaves can be controlled steplessly.
The primary sheave and the secondary sheave are each generally composed of a fixed flange and a moving flange which have a V-groove therebetween. The moving flanges are movable along the axis of the primary shaft or the secondary shaft. The speed change ratio can be controlled steplessly by moving the moving flanges by the groove-width adjusting mechanism.
Some types of V-belt type continuously variable transmissions move the moving flange of the primary sheave for groove-width adjustment by an electric motor. This type can flexibly adjust the groove width because the moving flange can be moved in either the direction to decrease the groove width of the primary sheave (to Top) or the direction to increase the groove width (to Low) by the output (moving thrust) of the electric motor (refer to, e.g., Japanese Patent No. 3043061). The output of the electric motor is controlled by the power supplied from a control unit. The supply power is determined from the deviation of the actual position of the moving flange from a target position. Some V-belt type continuously variable transmissions control the supply power by a pulse-width modulation (PWM) signal (refer to, e.g., Japanese Patent No. 3399727).
The inventor has worked toward development of an electronic belt-type continuously variable transmission that controls speed change ratio electronically, and has found that movement of the moving flange of the primary sheave sometimes has a delayed response when vehicles are started from Low mode. The response delay at the Low start is caused by a shortage of thrust necessary for moving the moving flange, which in turn is due to a shortage of output of the electric motor. To solve the problem, the output of the electric motor must be increased sufficiently to overcome the response delay at Low start.
However, increasing the output of the electric motor at Low start causes another problem of overshooting a target moving flange position during shifting to Top or during downshift.
Referring to FIG. 8, the problem of overshooting that occurs when the output of the electric motor is increased is described. FIG. 8 is a schematic diagram of the fluctuations of the moving flange position when the output of the electric motor is controlled to Low start. Line 90 indicates a set target moving flange position, line 92 indicates an actual moving flange position, and line 94 indicates engine speed. The horizontal axis indicates time. FIG. 8 shows that when lines 90 and 92 are close to each other, the moving flange reaches the target flange position more smoothly.
Lines 90 and 92 agree with each other in Low during upshift. That is, the response delay of the moving flange at Low start is improved. However, in the region shifted to Top, a so-called overshooting phenomenon occurs in which line 92 fluctuates, that is, the moving flange overshoots the target moving flange position and returns to reach the target moving flange position. During downshift, overshooting occurs across the almost whole region, in which line 92 fluctuates significantly, lacking stability.
Such an overshooting phenomenon causes a so-called hunting phenomenon as indicated by line 94 in which fluctuation of the engine speed is repeated, resulting in significantly uncomfortable ride.